Fuel cell powerplants produce electric power by electrochemically consuming a fuel and an oxidant in one or more electrochemical cells. The oxidant may be pure oxygen or a mixture of gases containing oxygen, such as air. The fuel may be hydrogen.
Each fuel cell generally has electrodes for receiving the gases, such as an anode electrode for fuel and a cathode electrode for an oxidant. The cathode electrode is spaced from the anode electrode and a matrix saturated with electrolyte is disposed between the electrodes.
Each electrode includes a substrate. The substrate has a catalyst layer disposed on the side of the substrate which faces the electrolyte matrix. In some instances, an electrolyte reservoir plate is on the other side of the substrate and is capable of providing electrolyte through small pores to the substrate. These electrolyte reservoir plates may have channels or passageways behind the substrate for carrying a reactant gas, such as gaseous fuel to the anode and gaseous oxidant to the cathode. For example, these channels might extend between parallel ribs on the substrate side of the electrolyte reservoir plate. A separator plate on the other side of the electrolyte reservoir plate provides a barrier to the loss of electrolyte and prevents mixing of the fuel and oxidant gases in adjacent cells. Another acceptable construction is to have the electrode substrate act both as an electrolyte reservoir plate and as an electrode substrate with channels on the separator side of the substrate.
Generally, a stack of fuel cells and separator plates are used in performing the electrochemical reaction. As a result of the electrochemical reaction, the fuel cell stack produces electric power, a reactant product, and waste heat. A cooling system extends through the stack for removing the waste heat from the fuel cell stack. The cooling system has a coolant and conduits for the coolant. The conduits are disposed in cooler holders to form coolers within the stack. Heat is transferred by the cooler holders from the fuel cells to the conduits and from the conduits to the coolant.
The cooler holder must be electrically and thermally conductive and may be permeable to gas. An example of such a cooler holder is shown in U.S. Pat. No. 4,245,009 issued to Guthrie entitled "Porous Coolant Tube Holder for Fuel Cell Stack". Alternatively, the cooler holder might be impermeable to gas. An example of such a cooler holder is shown in U.S. Pat. No. 3,990,913 issued to Tuschner entitled "Phosphoric Acid Heat Transfer Material". In Tuschner, the cooler holder serves the double function of cooler holder and separator plate.
Separator plates prevent the mixing of the fuel gas, such as hydrogen, disposed on one side of the plate, with an oxidant, such as air, disposed on the other side of the plate. Separator plates are, therefore, highly impermeable to gases such as hydrogen and highly electrically conductive to pass the electrical current through the fuel cell stack. In addition, separator plates must also tolerate the highly corrosive atmosphere formed by the electrolyte used in the fuel cell. One example of such an electrolyte is hot, phosphoric acid. In addition, separator plates, like cooler holders, must be strong, particularly in terms of flexural strength, which is a measure of the ability of the separator plate to withstand high pressure loads, differential thermal expansion of mating components, and numerous thermal cycles without cracking or breaking.
An example of a method for making separator plates for electochemical cells is discussed in U.S. Pat. No. 4,360,485 issued to Emanuelson et al., the disclosure in which is hereby incorporated by reference. In this method, the separator plate is formed by molding and then graphitizing a mixture of preferably 50 percent high purity graphite powder and 50 percent carbonizable thermosetting phenolic resin. In particular, Emanuelson discusses forming a well blended mixture of the appropriate resin and graphite powder. The mixture is then distributed in a mold. The mold is compacted under pressure and temperature to melt and partially cure the resin and to form the plate.
Electrolyte reservoir layers, such as are commonly found in electrolyte reservoir plates and as electrode substrates have requirements that differ from those for a separator plate. For example, reservoir layers must accommodate volume changes in the electrolyte during fuel cell operation. Examples of such electrolyte reservoir layers are shown in commonly owned U.S. Pat. Nos. 3,779,811; 3,905,832; 4,035,551; 4,038,463; 4,064,207; 4,080,413; 4,064,322; 4,185,145; and 4,374,906.
Several of these patents show the electrolyte reservoir layer as an electrode substrate. In addition to accommodating changes in acid volume due to electrolyte evaporation and changes in operating conditions of the cell electrode, substrates must satisfy several other functional requirements. For example, the substrate provides support to the catalyst layer and provides a means for the gaseous reactants to pass through the catalyst layer. The edges of the substrate are often required to function as a wet seal to prevent the escape of reactant gases and electrolyte from the cell. Finally, the substrate must be a good electrical conductor, a good thermal conductor and have adequate structural strength and corrosion resistance.
One material suggested for use in fuel cells, such as potassium hydroxide fuel cells, is discussed in commonly owned U.S. Pat. No. 4,064,207 issued to DeCrescente et al. entitled "Porous Carbon Fuel Cell Electrode Substrates and Method of Manufacture". DeCrescente suggests making the substrate from any inexpensive material available in filament form which can be pyrolized to form a carbon fiber. Examples of such filaments are filaments comprised of acrylonitrile polymers and filaments comprised of naturally occurring cellulosic fibers such as rayon. The carbonizable filaments are uniformly distributed on a planar support to felt the fibers. A resin binder is thereafter applied typically by spraying. Thereafter, the felt is subjected to pyrolysis by heating.
Another material commonly used as a reservoir layer in phosphoric acid electrolyte fuel cells is formed of carbon fibers bonded together with a resin such as a phenolic resin and heated to convert the resin and carbon fibers to graphite. Alternatively, carbon or graphite fibers may be bonded together with pyrolitic graphite by placing an amount of fibers in a decomposable hydrocarbon atmosphere (e.g., methane) under conditions which cause the hydrocarbon to break down into carbon and hydrogen. The carbon (now pyrolitic graphite) deposits on the fibers. These two materials are available commerically and are commonly referred to as carbon papers.
Although many precursor materials have been proposed for carbon fibers, only three, rayon, polyacrylonitrile (PAN), and pitch are used in commercial production. Such carbon fibers are flexible, lightweight, thermally and, to a large extent, chemically inert. These fibers are all considered to be good thermal and electrical conductors.
Low cost cellulose fibers, such as cotton fibers and wood fibers, are an attractive precursor material for carbon fibers. However, gas permeable components, such as fuel cell substrates, made from cellulose based carbon fibers have had lower electrical and thermal conductivities than electrode substrates made from pitch based carbon fibers. The lower electrical conductivity increases electrical resistance and causes an increase in fuel cell area to produce the same electrical power as smaller fuel cells manufactured from higher conductivity, pitch based carbon fibers. The lower thermal conductivity of fuel cell stacks containing cellulose based carbon fibers requires more coolers per fuel cell stack than fuel cell stacks made with pitch based precursor fibers. Finally, the carbon yield of cellulose fibers is greatly reduced by rapid heating rates of the fiber (greater than one-hundred degrees Fahrenheit per minute) requiring processing of these fibers at slower, less economical rates if a higher carbon yield is desired. As a result, fuel cell stacks using components made from cellulose based carbon fibers can cost more for a given amount of electrical power than fuel cell powerplants made from the more expensive pitch based fibers having higher electrical and thermal conductivities.
Accordingly, scientists and engineers are seeking to develop methods for making gas permeable fuel cell components, such as electrode substrates and carbon fibers for such components from cellulose precursor fibers at higher heating rates and with improved electrical and thermal properties.